1. Field of the Invention
The present invention relates to a driving device for a vibration type motor and, more particularly, to a driving device for a vibration type motor that changes driving conditions by a driving frequency.
2. Related Background Art
Conventionally, a driving device for a vibration type motor such as a vibration wave motor controls the rotation speed by changing the driving frequency, as disclosed in Japanese Patent Application Laid-Open No. 63-154074. Alternatively, the driving device controls the rotation speed by setting a plurality of driving voltages, selecting an arbitrary one in advance, and changing the driving frequency, or by setting an input power by PWM in advance in accordance with variations in driving voltage and changing the driving frequency.
In any of the above-mentioned driving devices for a vibration wave motor, the rotation speed of the vibration motor is controlled by changing the driving frequency during a series of operations from the start to end of driving. Driving is inefficient such that the power consumption is almost uniform regardless of the rotation speed or the input power increases for a lower motor rotation speed.
An element (output transistor such as a switching element) on the output side in the motor driving circuit must have a large output, which increases the mounting area and the cost.
When the driving frequency becomes lower than the resonant frequency, the conventional vibration wave motor suddenly stops. This problem is solved by the following method.
More specifically, the vibration state of the vibration member of the vibration wave motor is monitored to detect a shift from the resonant frequency. If the driving frequency comes near the resonant frequency, the driving frequency is generally changed not to be lower than the resonant frequency.
According to the conventional method for acheiving above, the arrangement for always detecting the above shift is provided.